Docs/Makefile: change 'which' to 'command -v'

The "which" command is not a posix command. Additionally, if a file
is not found, it outputs "command not found", so when checking to see
if the response is "", that doesn't work.

"command -v" is a posix compliant replacement that doesn't put out any
text if the command is not found, so is more suitable in this case
anyway.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I22207e818c847d50998f90c9abd55a3cb4bb2ab3
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72922
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
diff --git a/Documentation/Makefile b/Documentation/Makefile
index 836b6e5..8552c6d 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -10,9 +10,9 @@
 
 all: corebootPortingGuide.pdf
 
-SVG2PDF=$(shell which svg2pdf)
-INKSCAPE=$(shell which inkscape)
-CONVERT=$(shell which convert)
+SVG2PDF=$(shell command -v svg2pdf)
+INKSCAPE=$(shell command -v inkscape)
+CONVERT=$(shell command -v convert)
 
 codeflow.pdf: codeflow.svg
 ifneq ($(strip $(SVG2PDF)),)